Silicon carbide sintered product and method for production thereof

ABSTRACT

A silicon carbide sintered body, wherein the porosity obtained from areas of silicon carbide particles and silicon particles in a sectional polished surface of the silicon carbide sintered body is greater than 15% and less than 30%, when the porosity (%) equals (the area of silicon particles/(the area of silicon particles+the area of silicon carbide particles))×100; and a content of residual silicon is less than 4% to a total volume of the silicon carbide sintered body.

The application is based upon and claims the benefit of priority fromthe prior Japanese Patent Applications, that is, Japanese PatentApplication Publication Nos. 2002-328214, filed on Nov. 12, 2002 and2003-344849, filed on Oct. 2, 2003 submitted by the same patentapplicant; the entire contents of which are incorporated herein byreference.

TECHNICAL FIELD

The present invention relates to a silicon carbide sintered body and amanufacturing method thereof.

BACKGROUND ART

A silicon carbide sintered body has been applied variously; however, insome technical fields, a range of applications of the silicon carbidesintered body has been restricted. For instance, in the applicationwhere it is exposed to such a high temperature as 1420° C. that is themelting point of silicon or more, it is feared that residual silicon ina silicon carbide sintered body may elute. As a result, a range ofapplications of the silicon carbide sintered body has been restricted.

In order to overcome the foregoing problems, some technologies have beenproposed (for instance, patent documents 1 and 2).

Patent document 1: Japanese Patent Application Laid-Open No. 59-184768

Patent document 2: Japanese Patent Application Laid-Open No. 63-30386

However, since the foregoing problems have not yet been overcome, asmeans for further improving the heat resistance and the reliability ofthe silicon carbide sintered body, a reduction in an amount of theresidual silicon in the silicon carbide sintered body has been demanded.

Furthermore, in some technical fields, from viewpoints of making thevariation smaller in the mechanical characteristics, the electricalcharacteristics and the thermal characteristics of the silicon carbidesintered body, the uniform dispersibility of silicon particles in atexture of the silicon carbide sintered body has been demanded.

DISCLOSURE OF INVENTION

The present invention relates to items described below.

-   [1] A silicon carbide sintered body, wherein    -   the porosity obtained from areas of silicon carbide particles        and silicon particles in a sectional polished surface of the        silicon carbide sintered body is greater than 15% and less than        30%, when the porosity (%) equals(the area of silicon        particles/(the area of silicon particles+the area of silicon        carbide particles))×100; and    -   a content of residual silicon is less than 4% to a total volume        of the silicon carbide sintered body.-   [2] The silicon carbide sintered body according to the item [1]    above, wherein    -   a total content of impurity elements other than silicon and        carbon in the silicon carbide sintered body is less than 10 ppm.-   [3] The silicon carbide sintered body according to item [1] or [2]    above, wherein a content of nitrogen is greater than 150 ppm.-   [4] A manufacturing method of a silicon carbide sintered body that    uses a reaction sintering method, comprising    -   (1) dissolving and dispersing silicon carbide powder in a        solvent, followed by pouring an obtained slurry-like powder        mixture in a mold, further followed by drying to obtain a green        body, (2) calcining the obtained green body under a vacuum        atmosphere or an inert gas atmosphere at a temperature in the        range of 1200° C. to 1800° C. to obtain a calcined body 1, (3)        impregnating the obtained calcined body 1 with a carbon        source, (4) calcining a calcined body 2 impregnated with a        carbon source, (5) reaction sintering where the obtained        calcined body 2 is impregnated with molten metallic silicon and        free carbon in the calcined body 2 and silicon are reacted to        obtain a silicon carbide body, and (6) heating in a vacuum        atmosphere at a temperature in the range of 1450° C. to 1700° C.        for 30 to 90 minutes to remove unreacted silicon.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a state of dispersion of SiC particles andSi particles in a texture of a silicon carbide sintered body obtainedaccording to reference example 1.

FIG. 2 is a diagram showing a state of dispersion of SiC particles andSi particles in a texture of a silicon carbide sintered body obtainedaccording to example 2.

BEST MODE FOR CARRYING OUT THE INVENTION

The present inventors, after studying hard, found that when amanufacturing method in which a calcined body containing silicon carbideand carbon is impregnated with metallic silicon, followed by subjectingcarbon and silicon to the reaction sintering to obtain a silicon carbidesintered body further includes heating to remove unreacted silicon, theforegoing problems can be overcome. In what follows, the presentinvention will be further detailed. In the beginning, ingredients thatare used in the manufacture of a silicon carbide sintered body accordingto the invention will be explained.

(Silicon Carbide Powder)

As silicon carbide powder that can be used in the invention, α-, β-,amorphous-silicon carbide or a mixture thereof can be cited.Furthermore, in order to obtain a high purity silicon carbide sinteredbody, as a raw material silicon carbide powder, high purity siliconcarbide powder is preferably used.

A grade of the β-silicon carbide powder is not particularly restricted.For instance, commercially available β-silicon carbide powder can beused. A particle diameter of the silicon carbide powder is, from aviewpoint of obtaining a higher density, preferably smaller.Specifically, it is in the range of substantially 0.01 to 10 μm, andmore preferably in the range of 0.05 to 5 μm. When the particle diameteris less than 0.01 μm, in measuring and mixing processes, it can behandled with difficulty. When it exceeds 10 μm, a specific surface areabecomes smaller, that is, a contact area with adjacent powder becomessmaller, unfavorably resulting in difficulty of obtaining higherdensity.

The high purity silicon carbide powder can be obtained, for instance, bya process in which a silicon source that contains at least one kind ofsilicon compound, a carbon source that includes at least one kind oforganic compound that generates carbon upon heating and a polymerizingor cross-linking catalyst are dissolved in a solvent, followed bydrying, further followed by sintering the obtained powder under anon-oxidizing atmosphere.

As the silicon source including the foregoing silicon compounds(hereinafter, referred to as a “silicon source”), liquid one and solidone may be used together; however, at least one kind of liquid one hasto be selected. As the liquid one, polymers of alkoxysilane (mono-, di-,tri-, tetra-) and tetra-alkoxysilane are used. Among the alkoxysilanes,tetra-alkoxysilane is preferably used. Specifically, methoxysilane,ethoxysilane, propoxysilane, buthoxysilane and the like can be cited.From a viewpoint of the handling, ethoxysilane is preferable.Furthermore, as the polymer of tetra-alkoxysilane, low molecular weightpolymers having the degree of polymerization of substantially 2 to 15(oligomers) and liquid silicate polymers further higher in the degree ofpolymerization can be cited. As solid ones that can be used incombination with these, silicon oxide can be cited. In the foregoingreaction sintering, the silicon oxide includes, other than SiO, silicagel (colloidal ultra-fine silica-containing liquid including insidethereof a OH group and alkoxy group), silicon dioxide (silica gel, finesilica, quartz powder) and the like. These silicon sources may be usedsingularly or in combination of two or more kinds.

Among these silicon sources, from viewpoints of the excellenthomogeneity and handling convenience, an oligomer of tetra-ethoxysilaneand a mixture of an oligomer of tetra-ethoxysilane and fine powderysilica are preferable. Furthermore, as these silicon sources, highpurity substances are used, ones of which initial impurity content is 20ppm or less is preferable, and ones of which initial impurity content is5 ppm or less is more preferable.

The polymerizing and crosslinking catalysts that are used to manufacturethe high purity silicon carbide powder can be properly selected inaccordance with the carbon sources. When the carbon source is a phenolicresin or a furan resin, acids such as toluenesulfonic acid,toluenecarboxylic acid, acetic acid, exalic acid, and sulfuric acid canbe cited. Among these, toluenesulfonic acid can be preferably used.

When the high purity silicon carbide powder that is raw material powderused in the foregoing reaction sintering method is manufactured, a ratioof carbon to silicon (hereinafter, abbreviated as C/Si ratio) can bedefined by applying the elemental analysis to a carbide intermediateobtained by carbonizing the mixture at 1000° C. Stoichiometrically, whenthe C/Si ratio is 3.0, free carbon in generated silicon carbide shouldbe 0%. However, in actuality, owing to sublimation of simultaneouslygenerated SiO gas, free carbon is generated at lower C/Si ratios. It isimportant that a compounding ratio is beforehand determined so that anamount of free carbon in the generated silicon carbide powder may not bean amount that is not appropriated for manufacturing a sintered body.Normally, in the sintering at approximately 1 atmosphere and at 1600° C.or more, when the C/Si ratio is set in the range of 2.0 to 2.5, the freecarbon can be suppressed; accordingly, this range can be preferablyused. When the C/Si ratio is set at greater than 2.55, the free carbonincreases drastically; however, the free carbon has an effect ofsuppressing the grain growth; accordingly, in accordance with anintended grain growth size, the C/Si ratio may be properly selected.However, when a pressure of the atmosphere is set lower or higher, theC/Si ratio for obtaining pure silicon carbide varies; accordingly, inthis case, the C/Si ratio is not necessarily restricted to the foregoingrange.

From the foregoing, as a method of obtaining particularly high puritysilicon carbide powder, a manufacturing method of high purity siliconcarbide powder that includes a manufacturing method of raw materialpowder described in a manufacturing method of a single crystal accordingto Japanese Patent Application Laid-Open No. 09-48605 previously appliedby the present applicant, that is, generating silicon carbide in whichwith at least one kind selected from high purity tetra-alkoxysilane andtetra-alkoxysilane polymers as a silicon source and with a high purityorganic compound that generates carbon upon heating as a carbon source,a mixture obtained by homogeneously mixing thereof is heated andsintered under a non-oxidizing atmosphere to generate silicon carbidepowder; and applying aftertreatment in which a process in which theobtained silicon carbide powder is kept at a temperature equal to 1700°C. or more and less than 2000° C., and during the temperature is kept, aprocess of heating at a temperature in the range of 2000° C. to 2100° C.for 5 to 20 min is applied at least once; wherein

-   -   by carrying out the foregoing two steps, silicon carbide powder        contents of the respective impurities in which are 0.5 ppm or        less can be obtained. Silicon carbide powder thus obtained is        irregular in the size thereof; accordingly, crushing and        classification are applied to make the size conform to the        foregoing particle size.

When nitrogen is introduced during silicon carbide powder ismanufactured, in the beginning, a silicon source, a carbon source and anorganic compound made of a nitrogen source and a polymerizing orcrosslinking catalyst are homogeneously mixed. As mentioned above, whena carbon source such as a phenolic resin, an organic compound made of anitrogen source such as hexamethylene tetramine and a polymerizing orcrosslinking catalyst such as toluenesulfonic acid are dissolved in asolvent such as ethanol, a silicon source such as an oligomer oftetra-ethoxysilane is preferably mixed sufficiently.

(Carbon Source)

A substance that is used as a carbon source is preferably a high purityorganic compound that has oxygen in a molecule and leaves carbon when itis heated. Specifically, a phenolic resin, a furan resin, an epoxyresin, a phenoxy resin, and various kinds of saccharides such asmonosaccharide such as glucose, oligosaccharides such as sucrose, andpolysaccharides such as cellulose and starch can be cited. From anobject of homogeneously mixing thereof with a silicon source, one thatis liquid at normal temperature, one that can be dissolved in a solvent,or one that softens or becomes liquid owing to heating such asthermoplastic or thermally melting one is mainly used. Among these, aresole type phenolic resin and a novolac type phenolic resin can bepreferably used. In particular, the resole type phenolic resin can bepreferably used.

(Silicon Source)

As a silicon source, at least one kind selected from high puritytetra-alkoxysilane, a polymer thereof and silicon oxide is used. In theinvention, the silicon oxide includes silicon dioxide and siliconmonoxide. As the silicon source, specifically, alkoxysilanes typical intetra-ethoxysilane, a low molecular weight polymer thereof (oligomer), asilicate polymer further higher in the degree of polymerization, and asilicon oxide compound such as silica gel and particulate silica can becited. As the alkoxysilane, methoxysilane, ethoxysilane, propoxysilaneand buthoxysilane can be cited. Among these, from the viewpoint of thehandling convenience, ethoxysilane is preferably used.

Here, the oligomer means a polymer having a degree of polymerization inthe range of substantially 2 to 15. Among these silicon sources, fromthe excellent homogeneity and handling convenience, an oligomer oftetra-ethoxysilane and a mixture of tetra-ethoxysilane and particulatesilica can be preferably used. Furthermore, as the silicon source, onehigh in the purity is used, that is, an initial content of theimpurities is preferably less than 20 ppm and more preferably less than5 ppm.

(Manufacturing Method of Silicon Carbide Sintered Body)

Subsequently, a manufacturing method of a silicon carbide sintered bodyowing to a reaction sintering method according to the invention will bedescribed with reference to embodiments.

An embodiment of a manufacturing method of a silicon carbide sinteredbody according to the present invention includes (1) dissolving anddispersing silicon carbide powder in a solvent, followed by pouring anobtained slurry-like powder mixture in a mold, further followed bydrying to obtain a green body, (2) calcining the obtained green bodyunder a vacuum atmosphere or an inert gas atmosphere at a temperature inthe range of 1200° C. to 1800° C. to obtain a calcined body 1, (3)impregnating the obtained calcined body 1 with a carbon source, (4)calcining a calcined body 2 impregnated with a carbon source, (5)reaction sintering where the obtained calcined body 2 is impregnatedwith molten metallic silicon and free carbon in the calcined body 2 andsilicon are reacted to obtain a silicon carbide body, and (6) heating ina vacuum atmosphere at a temperature in the range of 1450° C. to 1700°C. for 30 to 90 minutes to remove unreacted silicon. In what follows,regarding the embodiment of a manufacturing method of a silicon carbidesintered body, the respective steps will be detailed.

(1) Obtaining a Green Body

Silicon carbide powder and a defoamer are dissolved or dispersed in asolvent to manufacture a slurry-like powder mixture. In this case, froma viewpoint of homogeneously dispersing pores in a green body, it ispreferable to thoroughly agitate and mix. The agitation/mixing can becarried out by use of a known agitation/mixing unit such as a mixer anda planetary ball mill. The agitation/mixing is preferably carried outover 6 to 48 hours, in particular, over 12 to 24 hours.

As the silicon carbide powder that is used in the obtaining a greenbody, the foregoing silicon carbide powder can be cited. As the solvent,water, lower alcohols such as ethyl alcohol, ethyl ether and acetone canbe cited. As the solvent, one less in the content of impurity ispreferably used. As the defoamer, a silicone defoamer and the like canbe cited. Furthermore, when a slurry-like powder mixture is manufacturedfrom silicon carbide powder, an organic binder may be added. As theorganic binder, a deflocculant, a particulate adhesive and the like canbe cited. As the deflocculant, from a viewpoint of further improving aneffect of imparting the electrical conductivity, a nitrogen-basecompound such as ammonia and ammonium polyacrylate can be preferablyused. As the particulate adhesive, a polyvinyl alcohol urethane resin(such as water-soluble polyurethane) can be preferably used.

Next, after the slurry-like powder mixture is poured into a mold, leftand demolded, the solvent is removed by drying to manufacture a greenbody. In this case, in order to pour the slurry-like powder mixture intoa mold for molding, in general, cast molding is used. After theslurry-like powder mixture is poured in a cast molding die, left anddemolded, it is heated and dried under a temperature condition in therange of 40° C. to 60° C. or left drying naturally to remove thesolvent. Thereby, a green body having a specified dimension can beobtained. In the invention, the “green body” means a silicon carbidemolded body prior to subjecting to the reaction sintering, the moldedbody obtained by removing the solvent from the slurry-like powdermixture having a lot of pores inside thereof.

(2) Obtaining a Calcined Body 1

The green body is calcined to obtain a calcined body 1. The calcinationis carried out at a temperature in the range of 1200° C. to 1900° C.,preferably in the range of 1200° C. to 1800° C. and more preferably inthe range of 1500° C. to 1800° C. When the temperature is less than1200° C., since in the green body, silicon carbide particles are notsufficiently accelerated in coming into contact each other, the contactstrength becomes insufficient to result in inconvenience in thehandling. Furthermore, when it exceeds 1900° C., grain growth of thesilicon carbide powder in the green body becomes remarkable, resultingin, thereafter, causing insufficiency in penetration of molten highpurity silicon.

A temperature rising speed of the foregoing calcining is preferably inthe range of 1 to 3° C./min up to 800° C. and 5 to 8° C./min from 800°C. to the maximum temperature. A retention time at the maximumtemperature of the foregoing calcination is preferably in the range of10 to 120 min and more preferably in the range of 20 to 60 min. Theforegoing temperature rising speed and the retention time at the maximumtemperature can be properly determined in consideration of a shape and asize of the green body. The calcination is preferably carried out in avacuum atmosphere or an inert gas atmosphere from a viewpoint ofinhibiting oxidation. In the invention, the “calcined body 1” is asilicon carbide molded body before the reaction sintering is applied,which is obtained by calcining the green body, from which pores andimpurities are removed and which does not contain a carbon source. Onthe other hand, the “calcined body 2” described later is a siliconcarbide molded body before the reaction sintering is applied, which isobtained by calcining the foregoing calcined body 1 after the carbonsource is impregnated. Accordingly, it is obvious that the “calcinedbody 1” and the “calcined body 2” have to be discriminated. The bendingstrength of the calcined body 1 obtained in the foregoing (2) is, in apreferable mode, greater than 20 MPa.

(3) Impregnating a Calcined Body 1 with a Phenolic Resin

The calcined body 1 is impregnated with a phenolic resin as a carbonsource to manufacture a calcined body 1 impregnated with the phenolicresin. An impregnation method, as far as the calcined body 1 can beimpregnated with a phenolic resin, is not particularly restricted;however, it is preferable to impregnate a phenolic resin by making useof the capillary phenomenon. It is further preferable to impregnate thecalcined body 1 with a phenolic resin by making use of a cold isostaticpress (CIP). When the capillary phenomenon is utilized, the larger asize of a finally obtained silicon carbide sintered body, the larger adifference of densities of a periphery portion and a central portion;accordingly, a silicon carbide sintered body having homogeneous densitytends to be difficult to obtain. On the other hand, when the calcinedbody 1 is impregnated with a phenolic resin by use of the cold isostaticpress (CIP) method, even when a volume of a silicon carbide sinteredbody is large, a silicon carbide sintered body having homogeneousdensity can be obtained without limit. Accordingly, from a viewpointthat the calcined body 1 can be homogeneously impregnated with aphenolic resin without being affected by a size of a finally obtainedsilicon carbide sintered body, the cold isostatic press (CIP) method canbe preferably used.

When the calcined body 1 is impregnated with a phenolic resin as acarbon source by use of the cold isostatic press (CIP) method, by use ofa conventional cold isostatic press (CIP) processor, according to stepsbelow, the calcined body 1 can be impregnated with a phenolic resin.

In the beginning, the calcined body 1 and a phenolic resin as a carbonsource is poured into a flexible mold. After the mold is closely sealed,a phenolic resin is added into the flexible mold by an amount that is inexcess than a calculated value obtained by considering an actual carbonratio and can sufficiently immerse the green body. Specifically, it ispreferable to add in the flexible mold at a ratio of the calcined body1:the phenolic resin=1:3 to 6 (by volume ratio). As the foregoingflexible mold, one that can at least be closely sealed and applypressure on a substance accommodated in the mold from all directions andsimultaneously is used. Specifically, a rubber mold or a rubber glovecan be preferably used. Furthermore, as the phenolic resin, a liquidresole phenolic resin can be preferably used. Next, the sealed mold isplaced in a pressure chamber of a pressure vessel and a pressurizingliquid is filled in, followed by sealing with a plug of the pressurevessel. As the pressurizing liquid, a highly compressible liquid can beused. Specifically, from the viewpoints of higher compressibility andexcellent operability, water or a 30% boric acid aqueous solution can bepreferably used. Thereafter, by applying a cold isostatic press (CIP)process under predetermined conditions, the calcined body 1 isimpregnated with a carbon source. When the cold isostatic press (CIP)process is carried out, it is preferable that at room temperature, apressure is raised up to 1000 to 5000 kg/cm² over 1 hours, followed byholding the foregoing conditions for 0.5 hours. When the pressure is1000 kg/cm² or less, the impregnation becomes insufficient, and when itis 5000 kg/cm² or more, destruction may occur when lowering thepressure. More preferably, the pressure is raised up to 2500 to 3500kg/cm² over 2 hours, followed by holding under the foregoing conditionsfor 1 hour to apply the cold isostatic press (CIP) process. At thistime, it is preferable to lower the pressure over 2 hours after holdingunder the predetermined pressure.

By applying the cold isostatic press (CIP) process, a phenolic resin asa carbon source can be homogeneously impregnated in an entirety of thecalcined body 1, resulting in improving the purity of a silicon carbidesintered body that is obtained as a final product. In the invention, the“cold isostatic press (CIP) process (method)” means a processing methodthat, by making use of an equilibrium pressure or a hydrostaticpressure, homogeneously applies a high pressure on an entire surface ofthe molded body. In the cold isostatic press (CIP) process, other than aprocess that uses the liquid medium as a pressure medium, there is onethat uses a gas medium. As far as conditions of the foregoing coldisostatic press (CIP) process are satisfied, a processing method thatuses a gas medium can be used; however, from an economical point ofview, it is preferable to apply the cold isostatic press (CIP) processwith a liquid medium.

(4) Obtaining a Calcined Body 2

The calcined body 1 that is obtained by the foregoing (3) andimpregnated with a phenolic resin is calcined to obtain a calcined body2. Owing to the calcination, a carbon component that contributes to thereaction sintering can be obtained. The calcination is carried out at atemperature in the range of 900° C. to 1400° C., preferably in the rangeof 900° C. to 1200° C. and more preferably in the range of 950° C. to1100° C. When the temperature is less than 900° C., the carbonizationbecomes unfavorably insufficient. Furthermore, when the temperatureexceeds 1400° C., since the carbonization is over, it is unfavorablefrom an economical point of view. The temperature rising speed of thecalcination is preferably 2 to 4° C./min up to 600° C. and 8 to 10°C./min from 600° C. to the maximum temperature. However, it can beproperly determined by considering a shape and a size of the calcinedbody 1. The retention time at the maximum temperature of the calcinationis preferably in the range of 10 to 60 min and more preferably in therange of 20 to 30 min. However, it can be properly determined byconsidering a shape and a size of the calcined body 1. The calcinationis preferably carried out in a vacuum atmosphere or an inert gasatmosphere from a viewpoint of inhibiting the oxidation.

The bending strength of the calcined body 2 obtained in the foregoing(4) is greater than 20 MPa and, in a more preferable mode, greater than23 MPa. Thus, since the calcined body 2 has the mechanical strengthenough to tentatively mold, when the calcined body 2 is tentativelymolded, finally the moldability of a silicon carbide sintered body canbe improved. That is, an improvement in the mechanical strength of thecalcined body 2 enables to improve the moldability.

When the foregoing (3) impregnating the calcined body 1 with a phenolicresin and the (4) calcining are repeated, a conversion rate to SiCbecomes higher and thereby the mechanical strength of a finally obtainedsilicon carbide sintered body can be improved.

(5) Obtaining a Silicon Carbide Body

The calcined body 2 manufactured through the step (4) is heated in avacuum atmosphere or an inert gas atmosphere at a temperature equal toor more than a melting point of high purity metallic silicon,specifically, in the range of 1450° C. to 1700° C. to immerse in moltenhigh purity metallic silicon to manufacture a silicon carbide body(sintered body). When the calcined body 2 is immersed in the moltenmetallic silicon, liquefied silicon permeates into pores in the calcinedbody 2 due to the capillary phenomenon to react with free carbon in thecalcined body 2. Owing to the reaction, silicon carbide is generated andthereby the pores in the calcined body 2 are filled by the generatedsilicon carbide.

A reaction between silicon and free carbon, as shown in the step ofmanufacturing silicon carbide powder, occurs at a temperature equal toor more than the melting point of silicon; accordingly, molten highpurity metallic silicon heated to a temperature in the range of 1450° C.to 1700° C., after permeating into the calcined body 2, reacts with thefree carbon. Furthermore, a time period during which the calcined body 2is immersed in the molten metallic silicon is not particularlyrestricted and can be properly determined depending on a size and anamount of the free carbon in the calcined body 2. The high puritymetallic silicon is heated to 1450° C. to 1700° C., preferably to 1550°C. to 1650° C. to melt. When a melting temperature is less than 1450°C., because the viscosity of the high purity metallic silicon goes upunfavorably, the capillary phenomenon becomes difficult to permeate thehigh purity metallic silicon into the calcined body 2. Furthermore, whenthe melting temperature exceeds 1700° C., the vaporization becomesremarkable to unfavorably damage a furnace and the like.

As the high purity metallic silicon, powdery, granular and agglomeratemetallic silicon can be cited, and among these, 2 to 5 mm agglomeratemetallic silicon can be preferably used. In the invention, the “highpurity” means to be less than 1 ppm in the content of impurities.

As mentioned above, when the free carbon in the calcined body 2 andsilicon are reacted and thereby pores in the calcined body 2 are filledby generated silicon carbide, a silicon carbide sintered body high inthe density and excellent in the electrical characteristics can beobtained.

(6) Removing Unreacted Silicon

The silicon carbide sintered body manufactured through the step (5) isheated to a temperature equal to or more than the melting point ofmetallic silicon, preferably in the range of 1450° C. to 1700° C. andmore preferably in the range of 1600° C. to 1700° C. to remove unreactedsilicon. When the heating temperature is less than 1450° C., an amountof residual silicon increases and unreacted silicon seeps out on asurface of the silicon carbide sintered body. Furthermore, when theheating temperature is higher than 1700° C., the mechanical strength(MPa) of the silicon carbide sintered body deteriorates. In this case,as the heating time, it is preferable to keep for 30 to 90 min at theforegoing heating temperature, and more preferable to keep forsubstantially 60 min, for instance, 50 to 70 min.

Furthermore, when the unreacted silicon is removed under atmosphericpressure, unreacted silicon sublimated by heating tends to deposit on awork surface; accordingly, the unreacted silicon is preferably removedunder a vacuum atmosphere. Still furthermore, when high purity carbonwool or the like is disposed around to protect a furnace, the sublimatedsilicon reacts with the carbon wool and can be captured by the carbonwool.

As an optional step, in addition to the foregoing (1) through (6), ahydrofluoric acid process may be further disposed. When the hydrofluoricacid process is disposed to elute the unreacted silicon in hydrofluoricacid, the unreacted silicon that cannot be removed in the foregoing step(5) can be removed. A washing condition in this case is properlydetermined depending on a shape and size of the work. However, whentaking the operating efficiency and a time necessary for washing afterthe hydrofluoric acid process into consideration, it is preferable tocompletely remove the unreacted silicon in the step (6). When ultrasonicis combined in the washing, the washing efficiency can be furtherimproved.

(Silicon Carbide Sintered Body)

According the foregoing reaction sintering method, a silicon carbidesintered body that is high in the purity, density and toughness,electrically conductive and can be machined by the electric dischargemachining can be obtained. In the foregoing reaction sintering method,there is no particular restriction on a manufacturing device as far asit can satisfy the foregoing heating conditions of the invention; thatis, known heating furnaces or reaction devices can be used.

Thus obtained silicon carbide sintered body is less in an amount of theresidual silicon. Furthermore, the foregoing silicon carbide sinteredbody has a structure in which silicon carbide particles are uniformlydispersed. That is, the porosity of the silicon carbide sintered body isless than 30%. The porosity of the silicon carbide sintered body isgreater than 10% and less than 30%, and preferably greater than 15% andless than 20%. When the porosity exceeds the above-mentioned upperlimit, an amount of the residual silicon increases, and the mechanicalstrength of the silicon carbide sintered body is likely to decrease. Anamount of the residual silicon in the silicon carbide sintered body isless than 30% by volume of the silicon carbide sintered body.Accordingly, the heat resistance and the reliability of the siliconcarbide sintered body are improved, resulting in expanding a range ofapplications of products. The porosity in the invention means a valuethat is obtained according to an equation below after from a micrographof a polished sectional surface of a silicon carbide sintered body bymeans of the image processing areas of silicon carbide particles andsilicon particles are obtained.Porosity (%)=(area of silicon particles/(area of silicon particles+areaof silicon carbide particles))×100

Area ratios of silicon carbide and silicon of the silicon carbidesintered body (section/surface) are greater than 70% for the siliconcarbide area and less than 30% for the silicon area.

Furthermore, an amount of the residual silicon in the silicon carbidesintered body is less than 4%, preferably less than 2% to a total volumeof the silicon carbide sintered body. When it exceeds 4%, there is afear in that the residual silicon may elute during high temperature use.The lower limit of the amount of the residual silicon in the siliconcarbide sintered body is not particularly restricted; however, it issubstantially 0.5%. Since the reaction between Si and C causes a volumecontraction, it is difficult to make the amount of the residual siliconless than 0.5%.

The silicon carbide sintered body obtained according to the inventionhas the density of 2.9 g/cm³ or more and a structure in which mainlyisotropic silicon particles having an average particle diameter in therange of 2 to 8 μm are uniformly dispersed. Accordingly, it can be usedalso as a structural member small in the fluctuation of the density andthe like. It is reported that in general, when the density of thesintered body is less than 2.9 g/cm³, the mechanical characteristicssuch as the bending strength and the rupture strength and the electricalphysical properties deteriorate, and furthermore an increase inparticles increases contamination. Accordingly, the silicon carbidesintered body according to the invention can be said to have excellentmechanical and electrical characteristics. The density of the siliconcarbide sintered body according to the invention in a preferable mode isgreater than 3.0 g/cm³. Furthermore, when an obtained sintered body isporous, physical disadvantages such that the heat resistance, theoxidation resistance, the chemical resistance and the mechanicalstrength are poor, the washing is difficult, microcracks are caused andfragments become contaminant, and the gas permeability is generated arecaused, resulting in restricting applications. In the silicon carbidesintered body according to the invention, the foregoing disadvantagesdue to the porous body are difficult to occur.

A total content of the impurities in the silicon carbide sintered bodyobtained according to the invention is less than 1.0 ppm, preferablyless than 5 ppm, more preferably less than 3 ppm and still morepreferably less than 1 ppm. From a viewpoint of applying to thesemiconductor industrial field, the impurity contents according to thechemical analysis have meaning only as a reference value. From apractical point of view, evaluation differs depending on whether theimpurity distribution is homogeneous or locally predominant.Accordingly, in general, one skilled in the art, by use of a practicaldevice, variously evaluates whatever degree the impurity contaminates awafer under a predetermined heating condition. According to amanufacturing method in which a solid material obtained by homogeneouslymixing a liquid silicon compound, a nonmetal sintering additive and apolymerizing or crosslinking catalyst is heated to carbonize under anon-oxidizing atmosphere, followed by further sintering under anon-oxidizing atmosphere, a total content of impurities other thansilicon, carbon and oxygen contained in the silicon carbide sinteredbody can be made less than 1 ppm. A content of nitrogen in the siliconcarbide sintered body obtained according to the invention is greaterthan 150 ppm.

The silicon carbide sintered body thus obtained according to theinvention preferably has the following physical properties. The siliconcarbide sintered body according to the invention is less than 1 Ω cm inthe volume resistance, and, in a more preferable mode, in the range of0.5 to 0.05 Ω cm. In the silicon carbide sintered body according to theinvention, a total content of inevitable elements other than silicon andcarbon of the silicon carbide sintered body, that is, impurity elementsis less than 5 ppm. In the silicon carbide sintered body according tothe invention, the density is greater than 2.9 g/cm³, and in a morepreferable mode in the range of 3.00 to 3.15 g/cm³. In the siliconcarbide sintered body according to the invention, the bending strengthis greater than 200 MPa, and in a more preferable mode greater than 220MPa. The sintered body obtained according to the foregoing manufacturingmethod, according to applications, can be subjected to processes such asthe machining, polishing and washing. The sintered body according to theinvention can be manufactured by forming into a cylindrical sample(sintered body) followed by slicing in a diameter direction. As amachining method, the electrical discharge machining can be preferablyused. Resultant ones can be used in semiconductor manufacturecomponents, electronic information instrument components, opticalcomponents and the like.

As principal semiconductor manufacturing devices in which the sinteredbody components according to the invention can be used, an exposingdevice, a resist processor, a dry etching device, a washing device, aheater, an ion implanter, a CVD device, a PVD device, a dicing machineand the like can be cited. Examples of components include a plasmaelectrode for a dry etching device, a protector ring (focus ring), aslit component for an ion implanter (aperture), a protection plate foran ion generator and a mass analyzer, a dummy wafer that is used when awafer is processed in a heater and a CVD device, and a heat-generater ina heater, a CVD device and a PVD device, in particular a heater thatdirectly heats a lower portion of a wafer can be cited. As thecomponents for electronic information instrument, a disc base plate fora hard disc device, a thin film magnetic head base plate and the likecan be cited. Furthermore, as the optical components, a mirror forreflecting synchrotron radiation (SR), laser light and the like can becited.

In each of silicon carbide powder that is raw material powder of theinvention, a silicon source and a nonmetal sintering additive formanufacturing the raw material powder, and an inert gas that is used toobtain a non-oxidizing atmosphere, as the purity thereof, a content ofeach of the respective impurity elements is preferably 1 ppm or less.However, when these are within allowable limits of purification in theheating and sintering, it is not restricted thereto. Furthermore, theimpurity elements here include elements that belong to group I to groupXVI in the periodic table according to 1989 IUPAC inorganic chemistrynomenclature revised edition and have an atomic number of 3 or more,except for elements having an atomic number of 6 to 8 and 14 to 16.

In the foregoing, embodiments are illustrated and described; however, itis obvious that the present invention is not restricted to the foregoingembodiments.

EXAMPLES

In what follows, examples and comparative examples will be shown todetail the present invention; however, it is obvious that the presentinvention is not restricted to examples below.

Example 1

Preparation of Silicon Carbide Sintered Body

In accordance with the manufacturing method of the silicon carbidesintered body described in the foregoing detailed explanation, a siliconcarbide sintered body was manufactured under conditions shown below.

In the beginning, to 100 parts of high purity silicon carbide powder, asa silicon source, having a center particle diameter 5 μm (siliconcarbide that is manufactured according to a manufacturing methoddescribed in Japanese Patent Application Laid-Open No. 09-48605 and hasan impurity content of 5 ppm or less/1.5% by weight of silica), 40 partsof water, 0.3 parts of deflocculant and 3 parts of binder were added,followed by dispersing and mixing for 24 hours by use of a ball mill,and thereby a slurry-like powder mixture having the viscosity of 1 poisewas obtained.

The slurry-like powder mixture was cast in a plaster mold having alength of 60 mm, a width of 10 mm and a thickness of 5 mm, followed bynaturally drying at 22° C. for 24 hr, and thereby a green body wasobtained.

Next, the obtained green body was heated, in a graphite crucible havingan inner diameter of 200 mm and a height of 80 mm, under an argonatmosphere, to 1800° C. over 10 hours and calcined at the foregoingtemperature for 1 hour to obtain a calcined body 1.

Subsequently, as a phenolic resin, a resole type phenolic resin (Tradename “SK Light” manufactured by Sumitomo Chemical Co., Ltd.) was pouredin a rubber mold by an amount of six times a volume of a molded body,followed by applying a cold isostatic press process (CIP) at a pressureof 1.2 ton, and thereby the foregoing sintered body 1 was impregnatedwith the phenolic resin.

After the CIP process, the calcined body 1 impregnated with the phenolicresin was calcined similarly to the above at 1200° C. to obtain acalcined body 2. Next, by use of metallic silicon as a silicon source,at 1540° C., a Si impregnation process was carried out to obtain areaction sintered body. Furthermore, under a vacuum atmosphere, thereaction sintered body was heated to 1450° C., followed by keeping atthe temperature for 60 min to remove unreacted silicon, and thereby asilicon carbide sintered body was obtained.

Then, of the obtained silicon carbide sintered body, according tocriteria described below, the porosity, the residual silicon, theexudation, the mechanical strength, the average particle diameter, andthe density were observed. The processing temperature and processingtime conditions in the removing unreacted silicon, and obtainedexperimental results are shown in Table 1.

Examples 2 and 3 Comparative Examples 1 Through 4

Experiments were carried out similarly to example 1 except that theprocessing temperature and time in the removing unreacted silicon wereset at conditions shown in Table 1. The processing temperature and timeconditions in the removing the unreacted silicon and obtainedexperimental results are shown in Table 1. TABLE 1 Heating and removingcondition of unreacted silicon Example Example Example ComparativeComparative Comparative Comparative 1 2 3 example 1 example 2 example 3example 4 Condition Processing 1450 1600 1700 1400 1950 1600 1600temperature (° C.) Retention time (min) 60 60 60 60 60 20 100 ResultsPorosity (%) 30 29.6 28.9 32 35 33 34 Residual silicon (%) 3.9 3 2.1 51.5 4.8 1.7 Exudation None None None Yes None Yes None Mechanicalstrength 263 250 230 185 135 180 138 (MPa) Average particle 5.0 5.0 5.08.0 8.0 8.0 8.0 diameter of SiC particle (μm) Density (g/cm³) 2.95 2.952.95 2.86 2.86 2.86 2.86NotesWhen the exudation of silicon was observed after a sample was kept for30 min at 1500° C. under an argon atmosphere, it is noted as “Yes” andwhen exudation was not observed, it is noted as “None”.(Experimental Results)

From the foregoing experimental results, it was found as following.

Comparison between examples 1 and 3 and comparative examples 1 and 2:

According to examples 1 and 3, it is found that when the heating isapplied at a temperature in the range of 1450° C. to 1700° C. for 60min, a silicon carbide sintered body that does not exhibit the exudationof silicon and has sufficient mechanical strength can be obtained.

On the other hand, according to comparative examples 1 and 2, it isfound that even when the processing time is 60 min, when a processingtemperature is 1400° C., the exudation of silicon is observed and whenthe processing temperature is at 1950° C., a silicon carbide sinteredbody having insufficient mechanical strength can be obtained.

Comparison between Example 2 and comparative examples 3 and 4:

According to example 2, it is found that when the heating is applied at1600° C. for 60 min, a silicon carbide sintered body having excellentmechanical strength can be obtained without causing the exudation ofsilicon.

On the other hand, according to comparative examples 3 and 4, it isfound that even though the processing temperature is set at 1600° C.,when the processing time is 20 min, the exudation of silicon isobserved, and when the processing time is 100 min, though the exudationof silicon is not observed, a silicon carbide sintered body havinginsufficient mechanical strength can be obtained.

(Evaluation Criteria)

(1) Measurement of the Porosity (Surface Observation)

A section of the obtained silicon carbide sintered body was polished, ofa superficial layer at 0.5 mm from a surface of the section of thesilicon carbide sintered body, in a viewing field range of a rectangleof 340 μm×250 μm, by use of a digital image processor (trade name:LUZEX, manufactured by Nireco Corporation), the image analysis wasapplied. From areas of silicon carbide particles and silicon particlesin a sectional polished surface of the silicon carbide sintered body inthe foregoing viewing field, the porosity was obtained according to aformula, porosity (%)=(area of silicon particles/(area of siliconparticles+area of silicon carbide particles))×100.

(2) Residual Silicon (%)

Similarly to the measurement of the (1) porosity, a surface observationof the silicon carbide sintered body was carried out and the residualsilicon (%) was obtained by volume base.

(3) Exudation of Silicon

A silicon carbide sintered body was kept under an argon atmosphere at1500° C. for 30 min. Whether the exudation of silicon was caused on asurface of the silicon carbide sintered body or not was observed. Whenthe exudation of silicon was observed, it was evaluated as “Yes” and,when the exudation was not observed it was evaluated as “none”.

(4) Mechanical Strength

In accordance with JIS R1601, according to a three point bending test,the bending strength (MPa) of the silicon carbide sintered body wasmeasured.

(5) Average Particle Diameter

Similarly to the measurement of the foregoing (1) porosity, a surfaceobservation was applied to the silicon carbide sintered body and bymeans of the image analysis an average particle diameter (μm) of SiCparticles was obtained.

(6) Density

The density (g/cm³) was measured by an Archimedes' Method in accordancewith JIS R1634.

INDUSTRIAL APPLICABILITY

According to the present invention, the heat resistance and thereliability of a silicon carbide sintered body can be improved.

Furthermore, according to the invention, a silicon carbide sintered bodyhaving a structure in which silicon particles are uniformly dispersedcan be provided.

One skilled in the art can easily understand that what are mentionedabove are preferable embodiments of the present invention and manymodifications and corrections can be applied without deviating from aspirit and a range of the present invention.

1. A silicon carbide sintered body, wherein the porosity obtained fromareas of silicon carbide particles and silicon particles in a sectionalpolished surface of the silicon carbide sintered body is greater than15% and less than 30%, when the porosity (%) equals (the area of siliconparticles/(the area of silicon particles+the area of silicon carbideparticles))×100; and a content of residual silicon is less than 4% to atotal volume of the silicon carbide sintered body.
 2. The siliconcarbide sintered body according to claim 1, wherein a total content ofimpurity elements other than silicon and carbon in the silicon carbidesintered body is less than 10 ppm.
 3. The silicon carbide sintered bodyaccording to claim 1, wherein a content of nitrogen is greater than 150ppm.
 4. A manufacturing method of a silicon carbide sintered body thatuses a reaction sintering method, comprising dispersing silicon carbidepowder in a solvent, followed by pouring an obtained slurry-like powdermixture in a mold, further followed by drying to obtain a green body,calcining the obtained green body under a vacuum atmosphere or an inertgas atmosphere at a temperature in the range of 1200° C. to 1800° C. toobtain a calcined body 1, impregnating the obtained calcined body 1 witha carbon source, calcining a calcined body 2 impregnated with a carbonsource, reaction sintering where the obtained calcined body 2 isimpregnated with molten metallic silicon and free carbon in the calcinedbody 2 and silicon are reacted to obtain a silicon carbide body, andheating in a vacuum atmosphere at a temperature in the range of 1450° C.to 1700° C. for 30 to 90 minutes to remove unreacted silicon.
 5. Thesilicon carbide sintered body according to claim 1, wherein the bendingstrength is greater than 200 MPa.
 6. The silicon carbide sintered bodyaccording to claim 1, further comprising a structure in which siliconparticles are uniformly dispersed.
 7. The silicon carbide sintered bodyaccording to claim 1, wherein the porosity is greater than 15% and lessthan 20%.
 8. The manufacturing method of a silicon carbide sintered bodyaccording to claim 4, wherein in the heating to remove unreacted silicona temperature is kept in the range of 1600° C. to 1700° C. for 50 to 70minutes to remove the unreacted silicon.
 9. The manufacturing method ofa silicon carbide sintered body according to claim 4, wherein theobtained silicon carbide sintered body has the bending strength ofgreater than 200 MPa.
 10. The manufacturing method of a silicon carbidesintered body according to claim 4, wherein the obtained silicon carbidesintered body has a structure where silicon particles are uniformlydispersed.
 11. The manufacturing method of a silicon carbide sinteredbody according to claim 4, wherein the obtained silicon carbide sinteredbody has the porosity of greater than 15% and less than 20%.